The present invention relates to a method for reversibly switchable cross-linking of monomers.
During the manufacture of conventional polymers, an irreversible cross-linking takes place. During all bond-breaking reactions of these polymers, the structural elements of the polymers are irreversibly destroyed. In addition, the amount of energy needed for bond-breaking/destroying the known polymers is relatively large. Therefore, it is not possible to switch back and forth between polymerization and depolymerization.
U.S. Pat. No. 4,022,649 describes a metal laminate having high thermal stability which is produced by forming a cured film layer of a thermally stable heterocyclic polymer such as polyamide imide having only a small content of volatile compounds on at least one surface of one or both of metal plates or foils, for example, of aluminum, separately forming an adhesive layer of a thermally stable heterocyclic polymer having a thickness of one-third or less of the above mentioned film layer and containing a volatile matter in an amount of not more than 20% by weight, and then heat bonding both of the metal plates or foils through the above mentioned adhesive layer with the film layer being oriented toward the adhesive layer. The described polymer layer decomposes at temperatures above 300xc2x0 C.
Therefore, an object of the present invention is to provide a method which permits a reversibly switchable cross-linking of monomers.
This objective is achieved according to the present invention by a method for the reversibly switchable cross-linking of monomers, including the steps of
a) cross-linking of monomers by adding complexable metal ions;
b) bond-breaking of the polymer formed in step a) by applying a direct voltage, by changing the pH value, or by changing the temperature; the monomers including compounds of at least one of the general formulas: 
xe2x80x83R1 through R6 being able to be equal or different and being selected from the group consisting of H, OH, xe2x80x94NX2, xe2x80x94OX, xe2x80x94SH, xe2x80x94SX with X being selected from alkyl and/or aryl groups which are terminally substituted by reactive residues selected from the group consisting of OH, xe2x80x94CH(O)CH2, xe2x80x94NCO, xe2x80x94COOH and NZ2, with Z being selected from the group consisting of alkyl groups, aryl groups and a hydrogen atom, and one of the groups Z being a hydrogen atom with the proviso that not all residues R1 through R6 are hydrogen atoms at the same time; and
c) possibly, repeated cross-linking of the polymer bond-broken in step b) by applying a direct voltage opposite in polarity to that of step b), by changing the pH value, or by changing the temperature.
The complexable metal ions in step a) of the method according to the present invention may be selected from the group consisting of metal ions of groups II through XIV of the periodic system, in particular of iron, zinc, magnesium, titanium, aluminum, copper and cobalt ions.
In an embodiment of the method according to the present invention, the bond-breaking of the polymer formed in step a) and, possibly, the repeated cross-linking of the polymer bond-broken in step b) are carried out by applying a direct voltage of 0.5 to 1,000 V.
In another embodiment of the method according to the present invention, the bond-breaking of the polymer formed in step a) and, possibly, the repeated cross-linking of the polymer bond-broken in step b) are carried out by changing the pH to a pH value in the range from 2 to 13.
In yet another embodiment of the method according to the present invention, the bond-breaking of the polymer formed in step a) and, possibly, the repeated cross-linking of the polymer bond-broken in step b) are carried out by changing the temperature to a range from 30 to 250xc2x0 C.